1. Field of the Invention
The present invention relates to a radio communication system, a radio communication method, and a base station, in which a radio terminal transmits uplink user data to a base station via an enhanced dedicated physical data channel and the base station transmits transmission rate control data for controlling a transmission rate of the uplink user data to the radio terminal.
2. Description of the Related Art
Heretofore, a radio communication system including a base station and a radio network controller has been known. In such radio communication system, the base station has a single or multiple cells, and a radio communication is performed between each of the cells and a plurality of radio terminals. The radio network controller controls a plurality of base stations, and assigns a radio resource to the plurality of the radio terminals. Note that such a technique (hereinafter referred to as first technique) is sometimes referred to as R99 (Release 99) or the like.
For the purpose of improving the throughput and reducing the delay time, and the like, there has recently been proposed a technique in which a base station assigns the radio resources for uplink user data transmitted from each of the radio terminals to the base station (network side). Note that such a technique (hereinafter referred to as a second technique) is sometimes referred to as the high speed uplink packet access (HSUPA), the enhanced uplink (EUL) or the like.
Each of the cells functions as a serving cell or as a non-serving cell. A transport block size (TBS) is determined based on the transmission rate (for example, a scheduling grant (SG)) of the uplink user data, and is controlled by transmission rate control data transmitted from the serving cell and the non-serving cell. The transmission rate control data includes an absolute grant (AG) for controlling an absolute value of the transmission rate and a relative grant (RG) for controlling a relative value of the transmission rate (for example, see 3GPP T325.321 Ver. 7.5.0).
In this regard, in the above-described second technique, the base station stores, in the buffer, the uplink user data that is received from the mobile terminal via an enhanced dedicated physical data channel (E-DPDCH), and thereafter transmits the uplink user data to the radio network controller. Note that, the base station transmits the uplink user data to the radio network controller by use of a wired transmission path.
Incidentally, when the uplink user data stored in the buffer of the base station is discarded, a retransmission of the uplink user data is required. Similarly, when the uplink user data transmitted by use of the wired transmission path is discarded, a retransmission of the uplink user data is required.
When the above-described retransmission of the uplink user data is required to be performed, an amount of the uplink user data transmitted from the mobile terminal to the base station increases, thereby a delay of the uplink user data and a decrease of the throughput in the uplink user data occur.